THE CLIMATE AND WEATHER OF

AUCKLAND2nd edition P.R. Chappell

Note to Second EditionThis publication replaces the first edition of New Zealand Meteorological Service Miscellaneous Publication 115 (20), written in 1988 by J.W.D. Hessell. This edition incorporates more recent data and updated methods of climatological variable calculation.

NIWA SCIENCE AND TECHNOLOGY SERIES NUMBER 60

ISSN 1173-0382

© All rights reserved. The copyright for this report, and for the data, maps, figures and other information (hereafter collectively referred to as “data”) contained in it, is held by NIWA. This copyright extends to all forms of copying and any storage of material in any kind of information retrieval system.

While NIWA uses all reasonable endeavours to ensure the accuracy of the data, NIWA does not guarantee or make any representation or warranty (express or implied) regarding the accuracy or completeness of the data, the use to which the data may be put or the results to be obtained from the use of the data. Accordingly, NIWA expressly disclaims all legal liability whatsoever arising from, or connected to, the use of, reference to, reliance on or possession of the data or the existence of errors therein. NIWA recommends that users exercise their own skill and care with respect to their use of the data and that they obtain independent professional advice relevant to their particular circumstances.

THE CLIMATE AND WEATHER OF AUCKLAND2nd edition P.R. Chappell

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CONTENTSSUMMARY 6

INTRODUCTION 7

TYPICAL WEATHER SITUATIONS IN AUCKLAND 9Strong easterlies with rain 9Fine weather 9Southwesterly wind flows in winter 10Squally westerlies 10

CLIMATIC ELEMENTS 13Wind 13Sea breezes 15Rainfall 16

Rainfall frequency and intensity 19Recent extreme events in Auckland 20Periods of low rainfall 21

Temperature 22Air and sea temperatures 22Air temperature 23Earth Temperatures 25Frosts 26

Sunshine and Solar Radiation 27Sunshine 27Solar radiation 28UV (Ultra-violet radiation) 28

Fog 29Severe convective storms 30

Thunderstorms 30Hail 30Tornadoes 30

Sea swell and waves 30

DERIVED CLIMATOLOGICAL PARAMETERS 33Vapour pressure and relative humidity 33Evapotranspiration and soil water balance 34Degree-day totals 36

ACKNOWLEDGEMENTS 38

REFERENCES 38

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SUMMARYAuckland experiences a subtropical climate. The region lies some 13° of latitude south of the Tropic of Capricorn, so tropical plants which are protected for the winter months will flower and fruit in the summer, and cold climate vegetables planted in autumn will mature in early spring – providing the ground is well-drained. Almost any plant can be grown in Auckland providing the location is optimised with regard to radiation, shelter, drainage, and irrigation. Summers tend to be warm and humid, while winters are relatively mild, and many parts of the region only receive a few frosts each year. Rainfall is typically plentiful all year round, with sporadic very heavy falls. Dry spells may occur during the summer months, but they are usually not long-lived. Most parts of Auckland receive around 2000 hours of bright sunshine per year. Sometimes Auckland experiences extreme events that cause flooding and wind damage, but generally these events are not as severe as in other regions.

Figure 1. The Auckland region, showing places mentioned in the tables, figures, and text.

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INTRODUCTIONThe mean high pressure belt in the New Zealand sector of the Southern Hemisphere is centred near 30° S so that westerly winds predominate over the country. On a day to day basis, however, there is great variability in the pressure distribution, and sometimes intense anticyclones occur to the south of the country with depressions to the north, causing an easterly flow with the reversal of the usual weather pattern. These blocking situations may be rather persistent and interrupt the more common westerlies associated with the eastward progression of weather systems.

The Auckland region, lying in the northern part of the country, and north of the main mountain chains and the volcanic plateau, is less vulnerable to outbreaks of Antarctic air than most of the country. However, it is one of the first areas to encounter storms of tropical origin. Auckland’s rain occurs mainly with northerly winds. The region is relatively warm and the higher absolute humidities give the climate a different character to that of the southern and eastern regions.

The Auckland region (i.e. the area administered by the Auckland Council) has an indented coastline including three major harbours (Kaipara, Waitemata, and Manukau). The Auckland isthmus is less than 2 km wide at its narrowest point. There are two dissected plateaux rising to 480 m and 690 m (Waitakere and Hunua Ranges1, respectively) and numerous islands – the largest being Great Barrier Island (285 km2). Great Barrier, along with islands in the inner Hauraki Gulf (e.g. Waiheke Island), as well as the Coromandel Peninsula (which is not in the Auckland region), shelter the waters of the eastern seaboard. The Hauraki Gulf is consequently a popular area for maritime activities. The west

1As of 2010, the Hunua Ranges are outside the jurisdiction of Auckland Council (now part of Waikato Region), but have been included in this publication.

coast of the region is exposed to ocean swells emanating from the Tasman Sea and the Southern Ocean, which may be further disturbed by the prevailing southwest winds. These black sand coasts present a startling contrast to those of the east. Figure 1 provides geographic context for the Auckland region, and shows all locations mentioned in the text and following tables.

All numbers given in the following tables are calculated using data from the 1981-2010 normal period (a normal is an average or estimated average over a standard 30 year period), unless otherwise stated.

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TYPICAL WEATHER SITUATIONS IN AUCKLANDStrong easterlies with rainAuckland’s heaviest rainfalls occur when there is a depression to the north or northwest with a strong north to northeast wind flow over the city, and a front embedded in the flow. Such a situation occurred on July 15, 1987 (Figure 2). The usual pattern of anticyclones to the north of the country and depressions to the far south is reversed on this occasion. An intense anticyclone is centred southeast of the South Island, and a deep, complex depression lies to the north of the country, with a strong north-easterly flow lying between the two pressure systems. These are prevented from moving rapidly by another anticyclone to the east, acting as a block. The frontal zone demarks the boundary between air arriving from the Cook Islands region and that arriving from the south Tasman Sea. Surface winds over Auckland reached 50-60 km/hr with winds of 80 km/hr at 1000 m altitude. Very heavy rain was recorded on the Coromandel Peninsula (up to 200 mm) and rainfall over Auckland during the episode totalled 43 mm. Flooding occurred in South Auckland about the Hunua Ranges and there was also some wind damage.

Fine weatherAuckland’s sunniest days occur during anticyclonic conditions in a light southeasterly flow, as on 31 January 1985 (Figure 3). Due to minor causes (e.g. sea breeze convergence zones or an anticyclonic subsidence inversion) local variations in wind direction and cloud cover may occur, but fine weather predominates. On the day of this example, over twelve hours of sunshine were recorded in Auckland city.

Figure 2. Strong easterlies with rain in Auckland.

Figure 3. Fine weather in Auckland.

Figure 4. Southwesterly wind flows in Auckland.

Figure 6a. Tropical cyclones which made landfall in New Zealand during December, 1970-2010. Source: Southwest Pacific Enhanced Archive of Tropical Cyclones (SPEArTC; Diamond et al., 2012).

Figure 6b. Tropical cyclones which made landfall in New Zealand during January, 1970-2010. Source: SPEArTC (Diamond et al., 2012).

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Southwesterly wind flows in winterDue to the tendency for anticyclones to form over Australia in winter, and for a major trough of low pressure to lie to the east of New Zealand, southwest airstreams are relatively common in the cooler half of the year. They are particularly persistent during periods when the Southern Oscillation Index is low (El Niño conditions). Southwesterlies produce cloudy, showery weather in Auckland, especially in winter when comparatively warm seas tend to destabilise the flow. Showers are more frequent in the hilly areas of the Waitakere and Hunua Ranges. The southwest flow is seldom undisturbed, and decaying cold fronts embedded in the flow frequently cause increases in the shower activity as they pass. Sometimes this is followed by a brief fine spell, but the cloudy, showery conditions quickly become re-established. Southwesterlies in summer are frequently fine, as the lower layers are not destabilised by comparatively warmer seas as they are in winter. Figure 4 is the synoptic analysis for a typical southwest day.

Squally westerliesAuckland is vulnerable to strong gusty westerlies which may be accompanied by thunderstorms, and rarely tornadoes. These conditions are most likely to occur in winter and spring. Such a situation is represented by Figure 5. Typical of these situations are strongly cyclonically-curved isobars with a strong wind flow rapidly decreasing towards the depression centre. Also typical are features in the upper atmosphere such as jet streams just to the north, and a column of very cold air to high levels. On this day thunderstorms were present over the Auckland, Taranaki, Waikato, and Bay of Plenty regions, and Auckland city experienced several tornadoes which damaged buildings and brought down power lines.

Figure 5. Squally westerlies in Auckland.

Figure 6c. Tropical cyclones which made landfall in New Zealand during February, 1970-2010. Source: SPEArTC (Diamond et al., 2012).

Figure 6d. Tropical cyclones which made landfall in New Zealand during March, 1970-2010. Source: SPEArTC (Diamond et al., 2012).

Figure 6e. Tropical cyclones which made landfall in New Zealand during April, 1970-2010. Source: SPEArTC (Diamond et al., 2012).

Tropical cyclones that reach Auckland and still retain very low pressures and hurricane force winds are very rare. However, other storms of tropical origin (which may never have been fully developed tropical cyclones) affect Auckland about once or twice each year, mainly between the months of December and April. They usually bring heavy rain and strong easterly winds.

Figure 6 shows, by months, the tracks of tropical cyclones which made landfall in New Zealand during the period between 1970 and 2010.

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Leigh 2 Whenuapai Airport

Manukau Heads Auckland Airport

Pukekohe EWS

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CLIMATIC ELEMENTSWindThe airflow over Auckland is predominantly from the southwest. This is particularly so in winter and spring, but in summer the proportion of winds from the northeast increases. This arises from the changing location of the high pressure belt, which is further south in summer and early autumn than it is in winter and spring. In addition, sea breezes add to the proportion of easterlies in eastern areas in summer and early autumn. Figure 6 shows mean annual wind frequencies of surface wind based on hourly observations from selected stations.

Mean wind speed data (average wind speeds are taken over the 10 minute period preceding each hour) are available for several sites in Auckland, and these illustrate the several different wind regimes of the region. Coastal areas (e.g. Auckland Airport) tend to be windier throughout the year compared with sheltered inland areas (e.g. Pukekohe). Table 1 gives mean monthly and annual wind speeds for selected stations in Auckland.

Table 1. Mean monthly/annual wind speeds (km/hr) for Auckland sites.

Location Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec AnnWarkworth EWS 14 13 13 12 14 14 15 14 16 17 16 16 14Whenuapai Airport 14 13 13 12 12 12 12 14 15 16 16 14 14North Shore ARC 10 10 9 8 9 9 9 9 10 12 12 11 10Onehunga ARC 12 12 11 9 10 10 10 11 12 14 14 13 12Auckland Airport 18 17 16 15 16 16 16 17 19 21 21 19 18Pukekohe EWS 9 8 8 8 8 8 8 9 9 10 10 9 9

Figure 7. Mean annual wind frequencies (%) of surface wind directions from hourly observations at selected Auckland stations. The plots show the directions from which the wind blows, e.g. the dominant wind direction at Auckland Airport is from the southwest.

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Spring is generally the windiest season throughout the region. Summer and autumn are the seasons when the greatest numbers of light wind days are recorded. Table 2 gives the seasonal proportion of strong and light winds as a percentage of the annual total. For example, of all strong winds recorded at Auckland Airport, 21% occurred in summer, 19% in autumn, 23% in winter and 36% in spring. In compiling this table a strong wind was defined as having a mean wind speed of at least 31 km/hr.

Table 2. Seasonal percentages of strong winds or calms (%) in Auckland.

Table 3. Average wind speed (km/hr) for selected hours in Auckland.

Table 4. Average number of days per year with gusts exceeding 63 km/hr and 96 km/hr, and gale force winds.

Table 5. Highest recorded gusts at selected Auckland stations, from all available data.

Location Summer Autumn Winter Spring

WarkworthStrong 22 15 24 40Light 25 26 25 23

Whenuapai AirportStrong 16 15 23 45Light 25 25 25 24

North Shore ARCStrong 10 11 30 49Light 25 25 25 25

Onehunga ARCStrong 21 5 37 37Light 25 25 25 25

Auckland AirportStrong 21 19 23 36Light 25 26 25 24

Pukekohe EWSStrong 40 0 40 20Light 25 25 25 25

Location 00 03 06 09 12 15 18 21Warkworth 11 11 11 13 19 20 17 13Whenuapai Airport 9 9 9 13 20 21 17 11

North Shore ARC 7 7 7 9 13 15 12 9Onehunga ARC 9 9 8 10 15 16 15 11Auckland Airport 15 14 14 16 20 23 22 17Pukekohe EWS 6 6 6 8 12 13 11 7

LocationGusts

>63 km/hrGusts

>96 km/hrDays of gale

Warkworth 90 7 3Whenuapai Airport 48 1 1Auckland Airport 55 3 3

Location Gust (km/hr) Direction (°) DateLeigh 2 150 200 14/06/1975Warkworth 131 050 22/05/1975Whenuapai Airport 115 310 19/07/1978North Shore ARC 95 240 08/10/2005Musick Point wind 148 - 02/08/1982Onehunga ARC 96 284 28/05/2002Auckland Airport 147 102 06/09/1981Manukau Heads 204 270 21/02/1992Pukekohe EWS 102 110 26/07/2008

Diurnal variation in wind speed is well-marked, with greatest wind speeds occurring in the early part of the afternoon. This is because at that time of day heating of the land surface is most intense and stronger winds aloft are brought down to ground level by turbulent mixing. Cooling at night generally restores a lighter wind regime. Table 3 gives average wind speeds at three-hourly intervals for selected stations.

Winds can be strong and gusty at times, especially in exposed coastal areas. Warkworth has the highest frequency of gusts per year that are greater than 63 km/hr and 96 km/hr. Warkworth and Auckland Airport show the same average number of days per year where gale force winds (10-minute average wind speeds in excess of 63 km/hr) are recorded, shown in Table 4. In comparison, Whenuapai Airport is more sheltered.

Although gale force winds can occur in any month, they are most frequent in winter. The highest gust recorded in the region was 204 km/hr at Manukau Heads (an exposed site on the west coast) on 21 February 1992. Maximum gusts recorded at different stations in the region are listed in Table 5.

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Sea breezesSea breezes are local onshore daytime winds generated on fine days by the sun warming the land surface more than the sea surface. In Auckland they occur most frequently between November and March when the sunshine is greatest and the wind flows are weakest. Sea breezes occur on approximately 20% of days during the summer months (McGill, 1987). Between 8 am and 10 am, breezes are initiated from the harbours in the region (Waitemata, Manukau, Kaipara) and along Auckland’s east coast, and in the late morning these ‘elementary’ breezes are augmented by ‘mature’ breezes from the main water bodies surrounding the region (Tasman Sea and outer Hauraki Gulf) (Figure 7). The breezes have speeds of less than 20 km/hr, and tend to weaken around 4 pm, and cease between 5pm and 10pm (McGill, 1987). The two breezes contribute towards the development of the sea breeze convergence zone, which is a

Figure 8. Typical sea breeze convergence zone; a) bay breezes from harbours at 11 am, b) mature breezes from oceans with sea breeze convergence zone at 2 pm.

band of cloud that occasionally contains scattered showers, that sits parallel to the coast over the Auckland isthmus. The direction of the large-scale, or synoptic, wind controls the position of the Sea Breeze Convergence Zone. If the synoptic wind about Auckland is from between northwest and southwest, the zone will move towards and sometimes into the Hauraki Gulf. This allows the west coast sea breeze, blowing from the southwest, to cover the isthmus. Should the synoptic wind be between north-northwest and east, the east coast sea breeze, blowing from the northeast, will move over the isthmus as the zone moves towards, or into, the Tasman Sea.

A B

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RainfallThe distribution of the Auckland region’s median annual rainfall is shown in Figure 8. Rainfall totals in the Hunua Ranges are about 50% higher than in lower-lying parts of Auckland. The Waitakere Ranges, Great Barrier Island, and the area around Warkworth also have higher rainfall totals than the urban Auckland and the east coast. Eastern areas such as Leigh and parts of Waiheke Island record on average about 900 mm of rain per year.

Seasonal influences on rainfall distribution are also quite well defined. Table 6 lists monthly rainfall normals and percentage of annual total for selected stations. This table shows a clearly defined winter rainfall maximum. Monthly percentages of the annual rainfall total are fairly consistent across the Auckland region, with around 32% of annual rainfall expected in the winter months from June to August, and around 20% of rain in the summer months from December to February.

Table 6. Monthly/annual rainfall normals (a; mm); percentage of annual total for each month (b; %).

Location Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Ann

Leigh 2a 73 70 98 82 105 117 130 120 95 83 69 76 1117

b 7 6 9 7 9 10 12 11 8 7 6 7

Warkworth Ewsa 86 88 109 107 134 155 180 152 131 108 92 113 1454

b 6 6 7 7 9 11 12 10 9 7 6 8

Whenuapai Airporta 85 73 102 91 87 107 142 140 109 96 95 105 1231b 7 6 8 7 7 9 12 11 9 8 8 9

Auckland, Albert Parka 84 61 96 92 91 106 114 127 100 86 91 89 1137b 7 5 8 8 8 9 10 11 9 8 8 8

Auckland, Owairakaa 74 66 91 102 109 124 147 116 103 101 90 93 1213

b 6 5 7 8 9 10 12 10 8 8 7 8

Auckland, Pakurangaa 82 64 94 103 98 113 138 129 108 95 100 90 1212b 7 5 8 9 8 9 11 11 9 8 8 7

Auckland Airporta 66 71 75 85 110 108 133 111 91 94 72 87 1101b 6 6 7 8 10 10 12 10 8 9 7 8

Figure 9. Median annual total rainfall for Auckland region.

The distribution of monthly rainfall is shown in Figure 9. The 10th percentile, 90th percentile, and mean rainfall values for each month are shown along with maximum and minimum recorded values for several stations.

Mean

Lowest Monthly Total

90 Percentile Value

10 Percentile Value

Highest Monthly Total

Leigh 2

010

020

030

040

050

0 Albany

010

020

030

040

050

0 Henderson River Park

010

020

030

040

050

0

Auckland Airport

010

020

030

040

050

0 Pukekohe EWS

010

020

030

040

050

0

Rain

fall

(mm

)Ra

infa

ll (m

m)

Rain

fall

(mm

)Ra

infa

ll (m

m)

Rain

fall

(mm

)

J F M J J N DA M A S O

J F M J J N DA M A S O J F M J J N DA M A S O

J F M J J N DA M A S O J F M J J N DA M A S O

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Table 6 continued.

Location Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Ann

Manukau Heads 2a 68 77 90 88 92 116 131 104 102 76 92 89 1124

b 6 7 8 8 8 10 12 9 9 7 8 8

Pukekohe Ewsa 73 64 76 86 122 141 151 144 111 120 98 98 1283b 6 5 6 7 10 11 12 11 9 9 8 8

Waiheke Island, Awaroa Valley

a 76 75 88 102 114 132 154 137 107 97 81 104 1266b 6 6 7 8 9 10 12 11 8 8 6 8

Port Fitzroy, Great Barrier

a 112 128 142 134 145 180 223 186 140 115 120 115 1740

b 6 7 8 8 8 10 13 11 8 7 7 7

Figure 10. Monthly variation in rainfall for selected Auckland stations.

Rainfall variability over longer periods is indicated by rainfall deciles, as given in Table 7. The 10th percentile values show the accumulated rainfalls that will normally be exceeded in nine out of ten years, while the 90th percentile values indicate the accumulated falls that will normally be exceeded in only one year in ten. The table includes periods from one month to twelve months; each period over one month begins with the

month stated. For example, using the table for Leigh, for three months it can be seen that in the three month period beginning in April, 181 mm or more of rainfall can be expected in nine years in ten, while a total of 443 mm or more should occur in only one year in ten.

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Table 7. Rainfall deciles for consecutive months

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecLeigh 21 month10th 23 14 19 24 27 64 56 49 54 39 27 2090th 128 142 181 161 239 208 210 189 164 146 117 1813 months10th 141 126 170 181 200 249 228 203 179 134 101 10290th 390 429 514 443 484 496 446 439 345 329 363 3736 months10th 382 407 478 482 486 470 420 404 305 293 320 33590th 726 870 922 838 795 765 737 698 636 662 598 66612 months10th 906 901 861 898 839 827 817 852 797 818 832 88490th 1441 1519 1474 1417 1376 1339 1350 1418 1395 1421 1372 1354Auckland Airport1 month10th 15 8 20 39 49 67 67 50 49 50 28 2890th 115 140 155 157 193 159 210 187 167 155 119 1463 months10th 92 145 203 200 250 245 209 213 175 176 124 13190th 383 336 381 399 471 483 472 373 347 329 396 4286 months10th 397 462 491 488 493 470 482 388 314 290 313 35590th 720 758 813 791 782 772 713 686 651 656 632 68912 months10th 902 890 850 797 812 877 895 880 862 847 882 86790th 1304 1324 1315 1280 1281 1289 1286 1289 1315 1300 1298 1275Pukekohe EWS1 month10th 13 8 22 28 69 96 66 55 59 47 41 3790th 149 151 170 150 182 221 241 216 219 177 164 1833 months10th 111 160 197 225 282 262 225 239 208 208 181 14890th 313 359 401 453 551 597 531 468 466 470 387 3606 months10th 423 450 456 472 491 477 486 426 421 359 349 40090th 783 917 995 939 943 927 854 751 771 620 665 64212 months10th 896 972 952 903 907 960 937 964 947 940 1001 99090th 1650 1648 1597 1563 1536 1466 1605 1629 1657 1557 1505 1545

Rainfall frequency and intensity

The average number of days each year on which 0.1 mm or more of rain is recorded varies from around 180 days in coastal areas of the region (e.g. Leigh) to over 210 days in inland areas (e.g. Warkworth). Table 8 lists the average number of days per month with 0.1 mm and 1 mm of rain for selected stations. The 0.1 mm rain days and 1 mm wet days show the same geographic variability.

Heavy rainfalls can occur with the passage of depressions of tropical origin over or close to Auckland, and with northeasterly flows between ridges of high pressure to the east and troughs over the Tasman Sea. Intense rainfalls also occur with thunderstorms. In Table 9, maximum short period rainfalls for periods of 10 minutes to 72 hours with calculated return periods are given for several

stations, from all available data. Also listed in this table are the maximum rainfalls expected in 2, 5, 10, 20, and 50 years. Depth-duration frequency tables for Auckland locations are available from NIWA’s High Intensity Rainfall Design System (HIRDS). HIRDS uses the index-frequency method to calculate rainfall return periods. For more information on methods and to use the tool, see www.hirds.niwa.co.nz.

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Table 8. Average monthly rain days and wet days for Auckland region; a: 0.1 mm rain day, b: 1 mm wet day.

Location Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Ann

Leigh 2a 11 11 12 13 17 17 19 20 18 15 13 12 179

b 8 7 8 8 11 13 13 14 12 11 9 8 121

Warkwortha 13 13 15 16 19 21 22 23 21 19 17 14 214b 9 8 9 11 12 15 16 16 16 13 12 9 146

Auckland Albanya 11 10 11 13 17 19 20 19 17 16 14 13 180b 8 7 8 10 12 15 15 15 13 12 10 9 133

Whenuapai Airporta 12 10 14 14 17 20 20 21 19 17 14 13 191b 8 7 9 9 11 15 16 15 14 12 9 9 135

Auckland Henderson River Park

a 12 12 13 16 20 22 22 23 20 19 16 15 209b 9 7 8 11 14 16 17 17 14 13 11 10 146

Auckland Owairakaa 12 10 12 15 18 21 22 21 19 16 15 13 194

b 8 7 8 11 12 15 16 15 13 12 10 9 136

Auckland Airporta 11 10 12 14 18 19 20 20 17 16 14 13 183b 7 7 8 9 13 14 15 14 12 12 9 9 129

Pukekohe EWSa 11 11 12 13 18 19 20 21 18 18 15 14 191b 8 8 8 10 13 15 16 16 13 13 11 10 141

Waiheke Awaroa Valleya 9 8 10 12 15 17 16 17 15 14 12 11 156b 7 7 8 10 12 14 14 15 12 11 10 10 130

Great Barrier RNZNa 12 13 14 16 19 20 21 20 17 15 15 12 195b 8 7 9 11 12 14 15 14 11 10 9 8 128

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Table 9. Maximum recorded short period rainfalls and calculated return periods from HIRDS.

Location 10min 20min 30min 1hr 2hrs 6hrs 12hrs 24hrs 48hrs 72hrsWarkworth a 15 23 26 39 59 109 178 201 263 264

b 7 9 7 9 16 26 56 22 48 25c 11 16 20 27 37 60 82 111 130 142d 14 20 24 34 47 76 104 142 166 182e 17 23 28 40 54 89 122 167 195 215f 19 27 33 33 63 104 142 195 229 251g 23 32 39 39 76 126 173 239 280 307

Whenuapai Airport a 17 28 36 58 104 176 176 260 297 298b 20 35 39 60 100+ 100+ 55 100+ 91 61c 11 16 20 29 38 57 74 95 112 124d 13 19 24 36 47 72 95 125 147 161e 15 22 28 41 55 85 112 149 175 193f 17 25 32 47 63 99 132 176 207 228g 20 30 38 56 76 121 163 219 258 284

Auckland Airport a 14 27 38 53 61 127 153 168.4 181 181b 9 47 95 90 43 100+ 100+ 48 40 29c 10 14 17 25 31 46 59 75 84 90d 12 18 21 30 39 58 75 97 110 118e 14 20 25 35 45 69 89 116 131 140f 16 23 28 40 52 80 105 137 154 166g 19 27 34 48 63 98 129 170 192 206

Pukekohe MAF a 18 20 27 32 57 142 161 167 172 172b 41 13 25 10 40 100+ 100+ 38 23 17c 10 13 16 23 30 46 61 80 94 102d 12 17 20 28 37 59 78 104 121 132e 14 19 23 32 43 68 92 123 143 157f 16 22 27 37 50 80 107 144 168 184g 19 26 26 44 60 97 131 178 208 227

a: highest fall recorded (mm)b: calculated return period of a (years)c: max fall calculated with ARI 2 years (mm)d: max fall calculated with ARI 5 years (mm)e: max fall calculated with ARI 10 years (mm)f: max fall calculated with ARI 20 years (mm)g: max fall calculated with ARI 50 years (mm)Recent extreme events in Auckland

Auckland has experienced numerous extreme weather events, with significant damage and disruption caused by flooding and high winds (e.g. Figure 10). The events listed below are some of the most severe events to have affected Auckland between 1980 and 2012.

17 July 1988: Torrential rain caused flooding in west Auckland, and a Civil Defence Emergency was declared (the first CDE in west Auckland for 15 years). Thirty-two people were evacuated from their homes. State Highway 16 was closed by flooding, and the railway from Auckland to Whangarei was closed due to slips. A teenage boy was presumed drowned after being swept away in Waiwera Estuary (north Auckland).

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21-22 January 1999: Heavy rain caused severe flooding in Pukekohe. The town received three times its normal January rainfall in just two hours on the 21st, an event with a return period of well over 150 years. Many homes were flooded and crops were destroyed. The floodwaters were 1 m deep in places. An elderly man drowned in a field after escaping from his car which was pinned to a fence by floodwaters. For six weeks after the flood, residents had to boil their water because of giardia contamination fears.

11-16 June 2006: High winds and rain battered Auckland, causing 700,000 people to be without power for a number of hours. Phone lines and cellphone networks were also affected, and the outage of more than 300 sets of traffic lights in central Auckland caused gridlock. The power cut also stopped and delayed trains, and hospitals cancelled all surgery.

9-11 July 2007: Torrential rain and hurricane-force winds hit Auckland during the height of this storm. 90,000 homes in the region were without power on the night of the 10th. Trains and ferries were cancelled or delayed, resulting in some commuters being unable to return home on the 10th. Two people were blown off their motorbikes on the Harbour Bridge, and the median lane barrier was moved out of alignment by the wind.

26-27 July and 29 July – 1 August 2008: Two large storms occurred within days of each other. On 26 July, about 60,000 customers were without power due to high winds bringing down power lines. More than 35 boats in the region broke free from their moorings. The second storm caused numerous slips, closing roads and damaging properties. The Muriwai Surf Lifesaving Club’s patrol tower had to be moved further back from the shoreline after the storms scoured the sand dunes and left part of the tower hanging over a drop of 10 m to the beach.

Periods of low rainfall

Periods of fifteen days or longer with less than 1 mm of rain on any day are referred to as “dry spells”. Dry spells are common in Auckland during the summer and early autumn. There is usually one, and frequently two, such periods each year between December and March. The average duration of a dry spell is about 20 days. The longest recent dry spell between three sites in Auckland (Leigh, Auckland Airport, and Pukekohe) was 34 days recorded in Pukekohe, from 22 January 1999 to 24 February 1999. During this dry spell, nine consecutive days were without any rain. Other long dry spells include 32 days at Leigh from 3 January 1988 to 3 February 1988, of which 15 consecutive days were without any rain, and the same dates at Auckland Airport, but at that site 16 consecutive days were without any rain.

Figure 11. Southbound lanes on Auckland’s northern motorway are submerged during a storm that coincided with high tide in January 2011. Source: NZTA

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Figure 12. Mean monthly land temperatures (Auckland Airport) and sea surface temperatures (east coast and west coast).

Figure 13. Monthly mean sea surface temperatures (°C) for: a) February; b) August, based on the years 1993-2002. Source: NIWA SST Archive, Uddstrom and Oien (1999).

Temperature

Air and sea temperatures

Auckland enjoys a mild climate with few extremes of temperature. Although this is partly due to the relatively low latitudes and elevations in the region, the extensive surrounding ocean also has a modifying effect on temperature in the region. Monthly mean sea surface temperature for the east and west coasts of the Auckland region is compared with mean monthly air temperature for Auckland Airport in Figure 11. There is a six to eight week lag between the minima of land and sea temperatures. Figure 12 shows the mean sea surface temperatures for the New Zealand region for February and August, which are the warmest and coolest months with respect to sea temperatures.

0

5

10

15

20

25

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Tem

pera

ture

(°C)

Month

Land temperature East coast SST West coast SST

MEAN FEBRUARY SST (°C) MEAN AUGUST SST (°C)

Leigh 2 Henderson River Park

Auckland Airport Ardmore Aero

Tem

pera

ture

(0 C)

3020

100

Tem

pera

ture

(0 C)

3020

100 Te

mpe

ratu

re (0 C

)

3020

100

Tem

pera

ture

(0 C)

3020

100

J F M A M J J A S O DN J F M A M J J A S O DN

J F M A M J J A S O DN J F M A M J J A S O DN

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Figure 15. Monthly variation in air temperatures for selected Auckland stations.

Air temperature

Most of the Auckland region experiences mean annual temperatures between 14 °C and 16 °C, with eastern areas generally warmer than western areas. Lower mean annual temperatures are experienced over higher elevations (e.g. Hunua Ranges; 12°C) due to the decrease of temperature with altitude. There is a deal of variability about this figure with high ground being relatively colder under windy conditions, while on cold nights hill tops may be warmer than low ground because of cold air drainage. The areal variation of annual median average temperature is shown in Figure 13.

Figure 14. Median annual average temperature for Auckland region.

Highest Recorded

Mean

Mean Daily Maximum

Mean Daily Minimum

Mean Monthly Minimum

Mean Monthly Maximum

Lowest Recorded

24

Compatible with the proximity to the sea and vulnerability to sea breezes, no great extreme maxima have been recorded. The highest temperature recorded in Auckland is 34.0°C at Lincoln Road, west Auckland, on 12 February 2009, and the lowest recorded is -5.7°C at Riverhead Forest in June 1936. These compare with national extremes of 42.4°C and -25.6°C.

Table 10. Average daily temperature range (Tmax–Tmin,°C).

Table 11. Mean hourly temperatures at Auckland Airport in January and July.

Figure 16. Mean hourly temperatures at Auckland Airport, January and July.

Location Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecLeigh 2 6 5.8 5.8 5.5 5.2 5.1 5.1 5.3 5.5 5.7 6 6Whenuapai Aws 9.3 9.6 9.6 9.1 8.7 8.5 8.8 8.2 8.2 8.1 8.3 8.8Auckland, Owairaka 8.1 7.8 8.1 8.1 7.4 7.5 7.7 7.5 7.6 7.4 7.6 7.6Auckland, Henderson, River Pk

11.2 11.2 11.2 10.7 9.7 9.6 9.9 10 10.2 9.9 10.1 10.4

Auckland Aero 7.6 7.6 7.7 7.3 6.8 6.8 6.8 6.7 6.7 6.5 6.7 6.9Pukekohe Ews 9.3 9.2 9.1 8.7 7.7 7.5 7.6 7.6 7.9 7.8 8.3 8.5

hrs 00 01 02 03 04 05 06 07 08 09 10 11January 17.9 17.7 17.4 17.2 17.1 16.9 17.0 18.1 19.2 20.2 21.0 21.6July 9.8 9.7 9.6 9.4 9.3 9.2 9.1 9.1 9.3 10.1 11.3 12.2

hrs 12 13 14 15 16 17 18 19 20 21 22 23January 22.1 22.4 22.5 22.5 22.2 21.5 21.0 20.2 19.2 18.8 18.6 18.2July 12.8 13.3 13.5 13.4 13.2 12.5 11.6 11.3 10.9 10.5 10.3 10.0

Many stations have not recorded any temperatures below freezing point. In particular, sites on high ground and near the coast (e.g. Leigh, Auckland Airport) almost never record temperatures as low as freezing point.

The annual mean daily temperature range for Auckland is small, averaging 7.9°C. Table 10 shows the average daily temperature range for each month for a number of sites in Auckland. Owairaka has the smallest temperature range for any station and Pukekohe has the largest.

Diurnal temperature ranges are also relatively minor. Table 11 and Figure 15 show mean hourly temperatures for Auckland Airport for January and July.

Figure 14 gives the monthly temperature regime (highest recorded, mean monthly maximum, mean daily maximum, mean, mean daily minimum, mean monthly minimum and lowest recorded) for selected sites in Auckland. Coastal sites (Leigh and Auckland Airport) show a smaller temperature range when compared to sites further inland (Henderson and Ardmore Airport). All sites show a winter minimum and summer maximum temperature pattern.

0

5

10

15

20

25

00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Mea

n ho

urly

tem

pera

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(°C)

Time

January July

25

Earth Temperatures

Earth temperatures are measured at varying depths and are important, amongst other things, for determining the growth and development of plants. Different plants have different rooting depths and earth temperatures are routinely monitored at 10, 20, 30, 50, and 100 cm depths.

Although earth temperatures are particularly sensitive to specific site conditions (aspect, elevation, soil colour and type, etc.) no great spatial variations in earth temperatures are apparent in Auckland. Fluctuations in earth temperatures are less than air temperatures due to the slower heating and cooling rates of the soil. Highest temperatures are found in January or February and lowest in July or August. Table 12 lists mean monthly earth temperatures for a number of standard depths. Figure 16 shows how earth temperatures change throughout the year for different depths at Auckland Airport. The temperature cycle for 100 cm depth is more damped than shallower depths.

Table 12. Mean 9am earth temperatures at different Auckland locations (°C), with site elevations in brackets

Figure 17. Average monthly 9 am earth temperatures for different depths and monthly mean air temperature at Auckland Airport.

Location Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec AnnWhenuapai Airport (26m)

10cm 19.2 19.3 17.5 14.5 11.5 9.7 8.4 9.4 11.0 13.1 15.8 17.9 13.920cm 20.7 20.9 19.1 16.2 13.2 11.2 9.8 10.7 12.1 14.1 17.0 19.2 15.330cm 20.4 20.7 19.3 16.7 13.9 11.8 10.4 11.1 12.4 14.3 16.9 18.9 15.6

Auckland, Owairaka (41m)

10cm 20.1 20.2 18.6 16.0 13.5 11.1 9.7 10.4 12.3 14.4 16.5 18.7 15.120cm 20.8 21.0 19.3 16.7 14.2 11.9 10.5 11.1 12.8 14.9 17.2 19.4 15.830cm 21.4 21.7 20.2 17.7 15.3 13.0 11.6 12.1 13.6 15.7 17.9 19.9 16.7

Auckland Airport (7m) 10cm 20.4 20.4 18.5 15.8 13.0 10.7 9.5 10.2 12.2 14.1 16.6 18.9 15.020cm 21.1 21.2 19.5 16.6 13.8 11.7 10.4 11.1 12.6 14.7 17.2 19.5 15.830cm 21.5 21.5 20.0 17.2 14.5 12.3 11.1 11.7 13.1 15.1 17.6 19.8 16.3

100cm 20.1 20.7 20.3 18.6 16.5 14.4 13.1 12.9 13.6 14.9 16.7 18.5 16.7

Pukekohe EWS (88m) 10cm 20.3 20.2 18.2 15.5 12.7 10.1 9.1 10.0 12.2 14.0 16.3 18.9 14.820cm 21.1 21.4 19.5 16.7 13.9 11.4 10.2 10.8 12.7 14.9 17.0 19.5 15.850cm 20.9 21.6 20.4 18.2 15.8 13.3 11.7 12.0 13.2 15.1 17.0 19.3 16.6

100cm 18.9 19.9 19.8 18.6 16.9 15.0 13.4 12.9 13.3 14.4 15.9 17.5 16.4

0

5

10

15

20

25

30

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Tem

pera

ture

(°C)

Month

10 cm 20 cm 30 cm 100 cm Air temp

26

Table 13. Occurrences of frosts and grass minimum temperatures in Auckland.

Frosts

Compared with many parts of the country, Auckland is mild and frosts are generally light and infrequent. Frosts only occur under very stable conditions and are accompanied by shallow inversions where temperature increases with height. Frost is a local phenomenon and its frequency of occurrence can vary widely over very small areas. Areas most likely to be subjected to frost are flat areas, where air is not able to drain away on calm nights, and valleys, where cold air is likely to drift from higher areas.

There are two types of frost recorded. Air frosts, when air temperature measured in a screen by a thermometer 1.3 m above the ground falls below 0°C,

Location Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec AnnLeigh 2 a 13.5 14.2 12.9 11.1 8.9 7.2 6.5 6.5 7.3 8.6 10.2 11.9 9.9

b 4.3 5.4 2.4 0.4 -0.5 -1.8 -2.0 -0.6 0.0 1.0 2.6 4.3 -2.0c 0.0 0.0 0.0 0.0 0.0 0.2 0.2 0.0 0.0 0.0 0.0 0.0 0.4d 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Henderson River Park a 11.7 12.0 9.9 7.8 5.9 3.5 2.3 3.0 4.7 6.9 8.6 10.7 7.3b 2.2 1.3 -1.3 -4.2 -4.7 -8.9 -8.3 -6.7 -4.7 -3.4 -0.6 0.8 -8.9c 0.0 0.0 0.0 0.5 2.2 6.3 7.9 6.4 3.4 0.8 0.0 0.0 27.4d 0.0 0.0 0.0 0.0 0.1 2.7 3.8 1.5 0.6 0.0 0.0 0.0 8.8

Auckland Airport a 13.8 14.2 12.4 9.6 7.6 5.7 4.9 5.6 7.1 9.0 10.7 12.8 9.5b 3.1 4.2 2.2 -0.8 -2.7 -4.4 -6.0 -4.2 -3.0 -1.8 1.5 2.5 -6.0

c 0.0 0.0 0.0 0.0 0.7 2.5 3.7 1.2 0.6 0.1 0.0 0.0 8.7d 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.1

Pukekohe EWS a 11.9 12.1 10.4 8.3 6.9 4.7 3.7 4.2 5.6 7.6 8.6 10.8 7.9b 2.4 0.6 0.1 -2.9 -2.2 -5.4 -5.0 -5.1 -3.3 -2.9 -1.4 0.0 -5.4c 0.0 0.0 0.0 0.1 0.5 4.2 4.9 2.5 1.2 0.5 0.0 0.0 13.9d 0.0 0.0 0.0 0.0 0.1 0.4 0.1 0.1 0.0 0.0 0.0 0.0 0.7

are rare in most parts of Auckland. Ground frosts are recorded when the air temperature 2.5 cm above a clipped grass surface falls to -1.0°C or lower. Ground frosts can be quite frequent in Auckland, especially in sheltered inland areas. Areas vulnerable to radiation fog are those most likely to suffer frost; whether fog or frost occurs is usually the result of the moisture content of the air. Table 13 lists for selected sites the mean daily grass minimum and extreme grass minimum temperatures and the average number of days each month with ground and air frosts. Data on air temperatures (mean daily, monthly minima and extreme minima) can be obtained from Figure 14.

a: mean daily grass minimum (°C)b: lowest grass minimum recorded (°C)c: average number of ground frosts per monthd: average number of air frosts per month

Leigh 2

010

020

030

00

100

200

300

010

020

030

00

100

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300

Auckland City

Auckland Airport Pukekohe MAF

Brig

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ine

(hou

rs)

Brig

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unsh

ine

(hou

rs)

Brig

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unsh

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(hou

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Brig

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(hou

rs)

J F M A M J J A S O N D J F M A M J J A S O N D

J F M A M J J A S O N DJ F M A M J J A S O N D

27

Sunshine and Solar Radiation

Sunshine

Most parts of Auckland receive about 2000 hours of bright sunshine per year (Figure 17). In general, central and eastern areas receive more bright sunshine than western and southern areas of the region, and islands in the Hauraki Gulf (e.g. Waiheke, Little Barrier, and Great Barrier Islands) receive even higher sunshine hours – over 2100 hours in some places. Southern areas around Pukekohe receive the least bright sunshine in the region. Figure 18 shows the monthly breakdown of bright sunshine experienced in Auckland, showing that it is cloudier during the winter months than in the summer.

Figure 18. Mean monthly sunshine for Auckland sites, and minimum and maximum sunshine hours recorded. These graphs were calculated from all available data.

Highest Recorded

Mean

Lowest Recorded

Figure 18. Median annual sunshine hours for Auckland, 1981-2010.

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Table 14. Mean daily global solar radiation (MJ/m2/day).

Table 15. Mean daily maximum UV Index at Leigh and Lauder.

Solar radiation

Solar radiation records are available for a number of sites in Auckland. Mean daily global solar radiation is presented in Table 14 for Leigh, Henderson, Auckland Airport, and Pukekohe. Insolation is at a maximum in December and January and a minimum in June.

Location Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec AnnLeigh 2 23 20 17 12 9 7 8 10 14 18 21 23 15Henderson, AKL 21 18 15 11 8 6 7 9 13 16 19 20 13Auckland Airport 23 20 16 12 8 7 7 10 14 17 21 22 15Pukekohe EWS 21 19 16 11 8 7 7 10 13 16 19 21 14

Location Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec AnnLeigh 12.2 10.7 8.4 5.1 2.8 1.8 1.9 3.0 4.8 7.2 9.8 11.5 6.6Lauder 10.4 8.9 6.0 2.9 1.3 0.8 0.9 1.7 3.3 5.2 7.9 10.0 4.9

UV (Ultra-violet radiation)

Ultra-violet radiation (UV) is recorded at two sites in Auckland, and the site with the most data is Leigh. Table 15 and Figure 19 show the mean daily UV Index at Leigh compared with Lauder, a site in the lower South Island. Leigh records higher UV levels than Lauder throughout the year due to Leigh’s northern location, although at both sites, summer months record significantly higher UV levels than winter months. Figure 20 shows an example of a UV forecast for Auckland city, and indicates the levels of UV where sun protection is required.

Figure 20. Mean daily maximum UV Index at Leigh and Lauder.

0

2

4

6

8

10

12

14

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

UV

Inde

x

Month

Leigh Lauder

29

Figure 21. UV Index forecast for Auckland city, January and July. Source: https://www.niwa.co.nz/our-services/online-services/uv-and-ozone

Table 16. Average number of days each year with thunder, fog, and hail, from all available data.

FogThe frequency of fog varies widely over the Auckland region. As the definition of fog is cloud at ground level with a horizontal visibility of 1000 m or less, the stratus cloud which forms on hill tops in rain must be regarded, and is sometimes called ‘high fog’. Areas predominantly affected by this type of fog are the Waitakere and Hunua Ranges.

Advection fogs, caused by warm moist air masses moving over cooler surfaces, are most common in late summer or autumn. They are associated with the tropical wet season, and mainly occur in the warm northeast winds ahead of decaying storms of tropical origin. Advection fogs can affect large areas and are deep enough to blanket most high ground. They can dominate the weather for one to three days and occur several times each year in the Auckland region, though mainly in autumn.

Location Thunder Fog HailLeigh 2 7 5 1Warkworth 5 23 8Whenuapai Airport 11 44 5Auckland, Albert Park 12 10 4Auckland, Owairaka 6 15 3Auckland, Henderson, River Pk 3 6 8

Auckland Airport 12 17 4Auckland, Ardmore 1 4 1Hunua Edl 4 16 15Port Fitzroy, Great Barrier 3 1 1

The third type of fog is that caused by radiation cooling in low lying areas where there is little wind. These radiation fogs have preferred locations where the air tends to pond, and usually occur in winter. The average number of days per year with fog for selected stations in the Auckland region is listed in Table 16. Favoured areas for fog formation in the Auckland region are Whenuapai (44 fogs per year on average) and Warkworth (23 fogs per year).

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concrete slabs on a construction site crushed their truck, and there were dozens of other injuries. On one street in Hobsonville, no houses escaped damage, with roofs and windows broken and trees felled. Hundreds of people were displaced by the tornado, 150 homes were damaged, and an emergency Civil Defence centre was set up at Whenuapai Airport. Damage costs were estimated at $11 million.

Sea swell and wavesIn enclosed waters such as the Waitemata, Manukau, and Kaipara Harbours, it is unlikely that the wind generated waves ever exceed two metres. This is because the winds to generate such waves would need to be either a steady wind of 70 km/hr or more (a very rare event in Auckland), or would require a much longer fetch than the enclosed harbours provide.

There is a known relationship between steady wind speed and wave heights over the open sea. The most probable wave heights for a given wind speed over a typical fetch length in New Zealand coastal waters of about 500 km are given in Table 17.

Much of the swell that affects the west coast of New Zealand originates in the ocean to the south of Australia. On the west coast of Auckland, the most frequent swell direction is from the southwest, occurring nearly 40% of the time (Gorman et al., 2003). The frequency of swells of less than one metre is about 20%, while swell over two metres occur approximately 35% of the time. Heavy southwest swells are particularly noticeable in winter and spring.

On the east coast of Auckland, swells from an easterly or northeasterly direction tend to predominate. These can originate from tropical cyclones well to the north of New Zealand or from anticyclones far to the east. Of all swells observed on the east coast the frequency of those less than one metre is about 40%, while for those greater than two metres is 8% (Gorman et al., 2003). The islands in the Hauraki Gulf form a buffer to large swells for the majority of the region.

Severe convective storms

Thunderstorms

In Auckland thunderstorms occur throughout the year, and have a maximum frequency in the winter months when cold, unstable air masses cross the region. Average annual frequencies for selected stations are given in Table 16, and range from 12 in Auckland city (Albert Park) and Auckland Airport to only one per year at Ardmore Airport. At some of the stations, it is likely that not all the thunderstorms are detected. The heavy rain, lightning, hail, wind squalls, and rare tornadoes which can occur with thunderstorms will sometimes cause severe local flooding, disruption of electrical and electronic equipment, and damage to trees, crops, and buildings.

Hail

Table 16 gives the average number of days per year on which hail is reported at selected stations. These range from 15 at Hunua to one at Leigh, Ardmore, and Great Barrier Island. As with thunderstorms, an unknown number of hail falls will escape detection at some of the stations. Hail is most likely over the six months from June to November.

Severe hailstorms are those containing stones with diameters of at least 0.5 cm or those which cause damage to crops. One such severe hail event occurred in November 1984, when parts of west Auckland near Kumeu experienced a hail storm of up to 30 minutes in duration. The storm, with hailstones up to 2.5 cm in diameter, caused significant damage to crops, glasshouses, sheds, trees, and houses. The estimated damage cost was over $10 million 2008 dollars.

Tornadoes

Tornadoes are rapidly rotating columns of air extending from the base of a cumulonimbus cloud, and have in New Zealand a damage path typically 10-20 m wide and 1-5 km long. The small size (compared to tornadoes in the USA), their short lifetimes and the sparse population of much of New Zealand, must result in an unknown number of tornadoes not being reported. During the period 1981-2012, 26 damage-causing tornadoes were reported in Auckland. One particularly severe tornado event was on 6 December 2012, when a tornado swept through the Hobsonville area near Whenuapai. Three people were killed when

31

Table 17. Generated wave heights associated with specific wind speeds. Assumes a fetch length of 500 km with unlimited wind duration.

Wind speed (km/hr) Associated wave height (m)10 0.520 130 240 350 475 7

100 11125 13+

DERIVED CLIMATOLOGICAL PARAMETERSApart from elements such as temperature and rainfall which can be measured directly, it has been found that parameters computed from several elements have some important uses especially in industry. Parameters which define the overall suitability of the climate for agriculture, horticulture, architectural and structural designs, and contracting, etc., are vapour pressure, relative humidity, evapotranspiration (leading to soil water balance), degree-days (thermal time), and rainfall extremes. Some of these parameters and their uses are discussed in the following paragraphs. Short-term high intensity rainfalls have been covered previously.

Vapour pressure and relative humidityVapour pressure and relative humidity are the two parameters most frequently used to indicate moisture levels in the atmosphere. Both are calculated from simultaneous dry and wet bulb thermometer readings, although a hygrograph may be used to obtain continuous humidity readings.

Vapour pressure is the part of total air pressure that results from the presence of water vapour in the atmosphere. It varies greatly with air masses from different sources, being greatest in warm air masses that have tropical origins and lowest in cold, polar-derived air masses. Vapour pressure can be important in determining the physiological response of organisms to the environment (very dry air, especially if there is a pre-existing soil moisture deficit, can cause or increase

Location Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec AnnLeigh 2 17.9 18.4 17.4 15.7 14.0 12.5 11.7 11.8 12.5 13.2 14.3 16.4 14.6Henderson River Park 18.2 18.6 17.1 15.8 13.7 11.5 10.8 11.6 12.9 13.6 14.6 16.7 14.6Auckland Airport 17.4 17.9 16.7 15.2 13.4 11.8 11.0 11.4 12.4 13.0 14.0 16.0 14.2Auckland Ardmore 18.3 18.6 16.9 15.2 13.4 11.6 10.9 11.4 12.9 13.9 14.7 17.0 14.6Pukekohe EWS 17.5 18.3 16.5 14.8 13.3 11.6 10.7 11.4 12.5 13.1 14.0 16.1 14.1

wilting in plants). Average 9 am vapour pressures for several stations are given in Table 18.

Relative humidity is high in all seasons throughout the region due to the influence of the surrounding sea and the lack of any large mountain masses. Table 19 gives the average relative humidity at 9 am for selected stations in Auckland. Most of the region shows similar relative humidity throughout the year, with Leigh generally experiencing the lowest average relative humidity for the region and Ardmore experiencing the highest average relative humidity.

As Auckland’s mean temperature is higher than in places further south and relative humidity is similar, Auckland has a somewhat higher vapour pressure than other main centres. The effect of this on people is what leads to Auckland’s climate being considered humid in comparison to other centres. Figure 21 shows how vapour pressure varies in the main centres across New Zealand.

Location Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec AnnLeigh 2 79 80 79 78 80 81 81 81 79 78 78 78 79Henderson River Park 80 85 86 89 91 92 92 91 85 82 78 78 86Auckland Airport 77 80 81 83 86 88 88 85 81 79 77 77 82Auckland Ardmore 86 88 87 89 91 92 92 91 88 88 84 85 88Pukekohe EWS 81 85 84 84 88 90 90 88 85 83 80 80 85

0

2

4

6

8

10

12

14

16

18

20

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

9 am

vap

our p

ress

ure

(hPa

)

Month

Auckland Wellington Christchurch Dunedin

33

Table 18. Mean monthly/annual 9 am vapour pressure (hPa) for selected Auckland stations.

Figure 22. Monthly average 9 am vapour pressures; Auckland, Wellington, Christchurch, Dunedin.

Table 19. Mean monthly/annual 9 am relative humidity (%) for selected Auckland stations

Evapotranspiration and soil water balanceEvapotranspiration is the process where water held in the soil is gradually released to the atmosphere through a combination of direct evaporation and transpiration from plants. A water balance can be calculated by using daily rainfalls and by assuming that the soil can hold a fixed amount of water with actual evapotranspiration continuing at the maximum rate until moisture depletion of the soil occurs. The calculation of water balance begins after a long dry spell when it is known that all available soil moisture is depleted or after a period of very heavy rainfall when the soil is completely saturated. Daily calculations are then made of moisture lost through evapotranspiration or replaced through precipitation. If the available soil water becomes insufficient to maintain evapotranspiration then a soil moisture

34

Location Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec AnnLeigh 2 DE 78 68 40 18 2 0 0 0 0 4 45 73 329

ND 16 15 11 8 1 0 0 0 0 1 10 16 79RO 3 0 10 8 28 53 89 72 35 10 0 2 309NR 0 0 0 1 2 6 9 9 4 1 0 0 32

Auckland Albany ND 78 74 41 15 2 0 0 0 0 1 27 55 293RO 16 16 12 7 1 0 0 0 0 0 6 12 70NR 9 4 7 7 33 84 120 81 44 22 3 1 415DE 0 0 0 1 3 10 12 10 5 2 0 0 46

Henderson River Park

RO 70 62 34 10 1 0 0 0 0 0 13 46 235NR 15 14 10 5 1 0 0 0 0 0 3 10 57DE 6 3 6 10 50 122 152 107 66 36 7 4 570ND 0 0 0 1 6 14 15 13 8 4 1 0 62

Auckland Airport NR 98 73 43 16 2 0 0 0 0 3 47 74 357DE 19 16 12 7 2 0 0 0 0 1 10 15 81ND 3 3 1 2 29 66 98 66 30 15 0 3 315RO 0 0 0 0 3 9 12 9 4 2 0 0 40

Pukekohe EWS DE 56 53 26 8 0 0 0 0 0 0 5 26 174ND 13 14 8 4 0 0 0 0 0 0 1 6 48RO 6 2 1 9 44 117 124 99 51 45 13 13 522NR 0 0 0 1 6 14 15 13 6 6 1 1 63

DE is the average amount of soil moisture deficit in mmND is the average number of days per month on which a soil moisture deficit occursRO is the average amount of runoff in mmNR is the average number of days per month on which runoff occurs

Table 20. Mean monthly/annual water balance summary for a soil moisture capacity of 150 mm

deficit occurs and irrigation becomes necessary to maintain plant growth. Runoff occurs when the rainfall exceeds the soil moisture capacity (assumed to be 150 mm for most New Zealand soils). The Auckland region is comparatively well served by frequent rainfalls in winter, but due to high evapotranspiration and a minimum of rainfall, soil moisture levels in summer are frequently such that irrigation or watering is necessary.

Mean monthly and annual water balance values are given in Table 20, for a number of sites in Auckland. It can be seen from this table that Auckland has about 11 days between November and April when there is insufficient soil moisture to maintain plant growth without irrigation, but this number varies between sites and between months. There is adequate moisture available to maintain plant growth between May and October. Figure 22 shows region-wide variability in days of soil moisture deficit per year.

Potential evapotranspiration (PET) has been calculated for Leigh, Auckland Airport, and Pukekohe, using the Penman method (Penman, 1948). The monthly mean, minimum, and maximum PET values are listed in Table 21.

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Table 21. Penman calculated maximum, mean, and minimum monthly average potential evapotranspiration (mm)

Location Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec AnnLeigh 2 Max 169 143 123 89 60 42 50 61 84 120 144 159

Mean 150 123 110 72 49 35 39 53 74 104 124 141 1074Min 117 103 89 56 38 29 30 47 59 82 104 118

Auckland Airport Max 187 146 124 79 51 36 45 60 81 126 150 176Mean 161 129 109 65 40 27 31 48 72 107 133 153 1075Min 137 113 91 52 33 18 23 35 61 87 116 139

Pukekohe EWS Max 155 112 98 61 37 25 26 43 61 93 119 135Mean 129 103 88 52 31 19 22 35 54 82 102 120 837Min 111 92 78 43 27 14 17 29 45 65 92 109

Figure 23. Auckland median annual days of soil moisture deficit, 1981-2010

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Table 22. Monthly/annual average growing degree-day totals above base 5°C and 10°C.

Degree-day totalsThe departure of mean daily temperature above a base temperature which has been found to be critical to the growth or development of a particular plant is a measure of the plant’s development on that day. The sum of these departures then relates to the maturity or harvestable state of the crop. Thus, as the plant grows, updated estimates of harvest time can be made. These estimates have been found to be very valuable for a variety of crops with different base temperatures. Degree-day totals indicate the overall effects of temperature for a specified period, and can be applied to agricultural and horticultural production. Growing degree-days express the sum of daily temperatures above a selected base temperature that represent a threshold of plant growth. Table 22 lists the monthly totals of growing degree-day totals above base temperatures of 5 °C and 10 °C for sites in Auckland.

Cooling and heating degree days are measurements that reflect the amount of energy that is required to cool or heat buildings to a comfortable base temperature, which in this case is 18°C. Table 23 shows that the number of cooling degree days reach a peak in summer in Auckland, where there is a higher demand for energy to cool building interiors to 18 °C. Conversely, heating degree days

Location Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec AnnLeigh 2 5°C 449 425 439 370 323 255 234 235 260 303 336 405 4032

10°C 294 284 284 220 168 105 80 81 110 148 186 250 2209

Whenuapai Airport 10°C 429 400 395 305 242 179 158 184 217 266 320 385 34815°C 274 259 240 156 91 48 32 44 71 112 170 230 1728

Henderson River Park

5°C 458 426 414 329 267 185 166 189 233 293 333 411 370410°C 303 285 259 180 113 52 36 48 87 138 183 256 1939

Auckland Airport 10°C 459 432 428 340 277 204 184 205 243 293 337 415 381710°C 304 290 273 190 123 64 45 56 94 138 187 260 2025

Pukekohe EWS 5°C 426 403 397 317 262 188 166 181 220 268 300 376 350410°C 271 262 242 168 109 52 32 39 74 113 150 221 1734

Figure 24. Median annual heating degree days for Auckland, 1981-2010.

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Table 23. Average cooling (CDD) and heating (HDD) degree-day totals with base 18 °C

Location Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec AnnLeigh 2 CDD 50 60 42 12 1 0 0 0 0 0 3 21 190

HDD 4 2 7 32 82 136 169 168 130 100 57 20 907

Whenuapai Airport HDD 39 42 23 3 0 0 0 0 0 0 3 18 129CDD 14 8 31 87 161 211 246 219 173 137 73 36 1396

Henderson River Park

CDD 63 63 32 8 1 0 0 0 0 1 6 31 205HDD 8 4 21 69 137 205 237 214 157 111 63 23 1250

Auckland Airport HDD 61 67 38 7 1 0 0 0 0 0 4 30 209HDD 5 3 13 57 126 186 219 198 147 110 57 18 1140

Pukekohe EWS CDD 40 44 23 5 1 0 0 0 0 0 2 15 129HDD 17 8 29 77 142 203 237 222 170 135 91 42 1373

reach a peak in winter, where the demand for energy to heat buildings to 18°C is highest. Figure 23 shows region-wide variability in the number of heating degree days per year. The number of heating degree days tends to be lower in low elevation coastal areas, compared with areas further inland and at higher elevations.

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ACKNOWLEDGEMENTSThe following people from NIWA are acknowledged for their assistance in preparing this publication: Dr Andrew Tait, James Sturman, Dr Elizabeth Somervell, Dr Michael Uddstrom, Dr Richard Gorman, Georgina Griffiths, and Erika Mackay.

Photo credits:Contents page, James Williams, NIWAPage 6, 12, 31, 34, 37 Petra Chappell, NIWAPage 8, 29, Uwe Duesing, NIWAPage 15, Reender BuikemaPage 21, Simon Williams, WeatherWatchPage 28, Erika Mackay, NIWA Page 32, Dave Allen, NIWA

REFERENCESNIWA databases used:

The National Climate Database www.cliflo.niwa.co.nz

HIRDS (High Intensity Rainfall Design System) www.hirds.niwa.co.nz

New Zealand Historic Weather Events Catalogue www.hwe.niwa.co.nz

NIWA Sea Surface Temperature Database

References:

DIAMOND, H. J., LORREY, A. M., KNAPP, K. R. & LEVINSON, D. H. 2012. Development of an enhanced tropical cyclone tracks database for the southwest Pacific from 1840 to 2010. International Journal of Climatology, 32, 2240-2250.

GORMAN, R. M., BRYAN, K. R. & LAING, A. K. 2003. Wave hindcast for the New Zealand region: Nearshore validation and coastal wave climate. New Zealand Journal of Marine and Freshwater Research, 37, 567-588.

MCGILL, A.J. 1987. Sea breeze circulations around Auckland, New Zealand Meteorological Service Scientific Report, 29: 40pp.

PENMAN, H. L. 1948. Natural evaporation from open water, bare soil, and grass. Proceedings of the Royal Society of London A, 193, 120-145.

UDDSTROM, M. J. & OIEN, N. A. 1999. On the use of high resolution satellite data to describe the spatial and temporal variability of sea surface temperatures in the New Zealand Region. Journal of Geophysical Research (Oceans), 104, 20729-20751.